A device called a microscopic mass spectrometry device or an imaging mass spectrometry device has been developed as a device for performing morphological observations of a sample such as biological tissue and simultaneously measuring the distribution of substances (molecules) present in a prescribed region of the sample (see Non-Patent Literature 1 and the like). With such a device, it is possible to obtain a distribution image (mapping image) of ions having specific mass/charge ratios m/z contained in an arbitrary region of the sample designated based on microscopic observations while almost completely maintaining the form of the sample without grinding or pulverizing the sample. In particular, the application of obtaining distribution information of specific components such as proteins contained in cells in the body, for example, is highly anticipated in the fields of biochemistry, medicine/pharmacology, and the like.
In order to make it possible for an analyzer to easily grasp desired information related to a sample—for example, the types of substances characterizing the sample, their concentration distributions, and the like—in imaging mass spectrometry, it is important to perform appropriate analytical processing on the collected mass spectrometry imaging data and to display the results in an appropriate format. When mass spectrometry imaging data of a two-dimensional region of a certain amount of area of a sample is obtained, the data contains mass spectrum data of multiple measurement points (micro areas). Therefore, the quantity of data becomes enormous. A great amount of time and effort are required for a person to examine this enormous amount of data and extract significant information.
In order to grasp the mass/charge ratios expressing a specific distribution in a certain measurement region, the mass/charge ratios of analytical target peaks are typically first selected by executing peak detection on the mass spectrometry spectrum data (see Non-Patent Literature 2). One known example of a peak detection method is a method in which a number of peaks designated in order of decreasing signal intensity in the mass spectrum are extracted and the mass/charge ratios of these peaks are analyzed. In this method, peak detection dependent on signal intensity alone is performed, so peaks with a signal intensity exceeding a certain level are all selected without monoisotopic peaks or isotope peaks derived from the same substance being differentiated from one another. Therefore, even if a prescribed number of peaks are selected from the mass spectrum, isotope peaks derived from the same substance are, in actuality, often included (that is, the selected peaks are essentially redundant). In order to avoid this, it is necessary to identify monoisotopic peaks and isotope peaks derived from the same substance in the mass spectrum or to identify isotope peak groups to which a plurality of isotope peaks derived from the same substance belong.
The method described in Patent Literature 1 is a method for identifying both monoisotopic peaks and isotope peaks derived from the same substance in a mass spectrum in which both types of peaks are present. This is a method in which the intensity ratios of a plurality of isotope peaks presumed to belong to a single isotope peak group in an actually measured mass spectrum and the intensity ratios of isotope peaks calculated theoretically based on the natural isotope ratios and the like of each element are compared, and after an isotope peak group assessed to have matching intensity ratios is selected, monoisotopic ion peaks are selected from the peaks contained in the isotope peak group. However, this method fundamentally performs processing on a single mass spectrum. Therefore, when this method is used for the analysis of mass spectrometry imaging data, it is necessary to repeat the same processing for each mass spectrum obtained for a single micro area, and this processing itself takes an enormous amount of time. Moreover, since the obtained results (m/z values of peaks) are obtained by means of processing performed separately in each micro area, it is necessary to coordinate the results (m/z values) between each micro area in order to handle the results as imaging data, which is troublesome. Further, depending on the measurement conditions, the intensity ratios of a plurality of isotope peaks may not be as predicted theoretically, and in such cases this method cannot be applied.
The method described in Patent Literature 2 is a method for identifying monoisotopic peaks/isotope peaks confined to mass spectrometry imaging data. In this method, mapping images of isotope peak candidates selected based on the intensity ratios of peaks in a mass spectrum are respectively created, and the similarity of the mapping images is assessed to determine isotope clusters to which isotope peaks derived from the same substance belong and monoisotopic peaks contained therein. However, since it is difficult to assess the similarity of a plurality of mapping images automatically, the person in charge of analysis must perform the operation of visually assessing the similarity of the images, but such assessment involves differences among individuals, so it is difficult to maintain the stability or objectivity of the results.                (PATENT LITERATURE 1) Japanese Unexamined Patent Application Publication 2006-284305        (PATENT LITERATURE 2) International Patent Publication 2008/126151 Pamphlet        